Spiral springs are employed in many industrial applications. Their principal characteristic relative to other types of springs is that the potential energy and spring constant characteristic of a spiral spring are confined to substantially a single plane, as opposed, for example, to torsion or compression springs in which the potential energy and spring constants thereof are reconfigured in three dimensions.
Spiral springs are often used in precise biasing applications in that the spring constant thereof can be used to oppose a force that may be applied to or against a positioning surface or ends of the spiral spring. Further, the spring constants of spiral springs can generally operate over longer linear displacements than is the case with other types of springs. As such, spiral springs have developed a recognized utility in a number of types of measurement and instrumentation systems when use over a larger linear displacement is desirable.
Notwithstanding the present widespread use of spiral springs, a problem which has accompanied (and limited) such usage has been a phenomenon known as spiral spring hysteresis. Such hysteresis refers to the effect known in spiral springs in which the spring constant of extension is not precisely equal to the spring constant of retraction of the spring. This phenomenon is understandable in that, in the operation of a spiral spring, the spring constant of extension is a result of the sum of a natural structural compressive force and a reactive force opposite to the pulling force on the spiral spring, whereas the spring constant of retraction is the sum of said natural compressive force and the smaller reactive force that is opposite the force of retraction. As such, the metallic crystalline domains of the spiral spring will not behave identically to the aggregate force of extension as they will to the smaller force of retraction. Further, the greater is the range of linear displacement (x) over which the spiral spring is employed, the more apparent will become the phenomenon of spring hysteresis, in that the force equation for springs takes the form F=-kx, where k is the spring constant. As such, the only known solution, in the prior art, to such hysteresis is to limit the range of displacement, in connection with extension and retraction, over which the spring is employed.
The inventors herein have found a spiral spring to have utility as a spring loading means of a drum or reel from which a measuring string or flexible wire may be extended or contracted as a part of a larger vector coordinate measurement apparatus. However, a difficulty which the inventors have observed in such use of a spiral spring is that small, but significant, differences in the measurement of coordinates are a function of whether a particular measurement by the string is made after the spiral spring has been in a condition of expansion or contraction, that is, whether or not the reactive force resulting from holding of the string has been at a higher or lower level, i.e., whether the aggregate compressive force on the spring has been at higher or lower level before a particular measurement. The present invention may therefore be viewed as a response to the above problem and as a solution thereto.
The inventors are not aware of any prior art which is relevant to their invention, as set forth herein.